Possible preserved eggs in Selkirkiid Worms from the Early Cambrian Xiaoshiba Lagerstätte of South China.

Over the past century the discoveries of numerous Cambrian Lagerstätten (such as the Burgess Shale and Chengjiang biotas) have given us remarkable insights into the functional anatomy, development, lifestyles and behaviour of the earliest Metazoan Animals. However, their reproductive anatomy and behaviour, for the most part remains unknown. Indeed, examples of reproductive behaviour from the entire Palaeozoic Era are rare, comprising a few strands and clusters of eggs, some scattered examples of brooding in Arthropods, and the possible embrio-structures seen in Orsten-type deposits from the Cambrian of Siberia and China. 

The Ecdysozoa is the single largest group of Animals today, comprising the vast Arthropod clade, plus worms such as the Nematoids and Scalidophorans (the group which includes the Priapulids). In Cambrian faunas this group seems to have been equally important, with Ecdysozoan Worms having apparently been a major part of all benthic communities, playing significant roles as bioturbators, predators and recyclers. The Scalidophorans are particularly well represented in the Burgess Shale, Chengjiang, and Qingjiang biotas.

Selkirkiid Worms are known from the Cambrian of North America, South China, and Greenland, and are probably stem group Priapulids (Worms more closely related to Priapulids than to any other living group, but which lived before the most recent common ancestor of all living Priapulids, or which are descended from such Worms but not the most recent common ancestor). They share several important morphological features with extant Priapulids, and other fossil Scalidophoran Worms, such as an eversible introvert lined with scalid rows and a circum-oral pharyngeal structure bearing teeth. However, Selkirkiids differ from Priapulids and other Scalidophorans in the presence of a conical tube, which with numerous, evenly spaced annuli, is open at both ends, encase the whole trunk, and presumably represents a cuticular structure secreted and renewed by underlying epidermal tissues. There are currently three named genera of Selkirkiids, Selkirkia from North America, Paraselkirkia from China, and Sullulika from Greenland, although it is likely that Paraselkirkia is a junior synonym of Selkirkia (i.e. these Worms should be placed in the same genus, and Paraselkirkia, having been used first, takes precedence and is the name which should be used.

In a paper published in the journal Geoscience Frontiers on 21 May 2021, Xiao-yu Yang of the Key Laboratory for Paleobiology and MEC International Joint Laboratory for Paleoenvironment at Yunnan University, Jean Vannier of the Université Claude Bernard Lyon 1, Jie Yang, also of the Key Laboratory for Paleobiology and MEC International Joint Laboratory for Paleoenvironment at Yunnan University, Deng Wang, also of the Université Claude Bernard Lyon 1, and of the Shaanxi Key Laboratory of Early Life and Environments and State Key Laboratory of Continental Dynamics at Northwest University, and Xi-guang Zhang, once again of the Key Laboratory for Paleobiology and MEC International Joint Laboratory for Paleoenvironment at Yunnan University, describe strucures which they interpret as eggs within the body cavities of the Selkirkiid Worm Paraselkirkia sinica, from the Early Cambrian Xiaoshiba Lagerstätte.

The fossils were collected from the Xiaoshiba section of the Hongjingshao Formation, roughly 3.7 km to the southeast of the village of Ala, in Kunming, Yunnan Province, and are dated to the Cambrian Stage 3 (known locally as the Canglangpuan), making them about 514 million years old. 

Paraselkirkia sinica is common in both the Chengjiang and Xiaoshiba biotas, being found at many locations in China, often in dense aggregations. Yang et al. collected about 200 specimens, all preserved as two-dimensional compressions, with pellet-like gut contents; this is fairly typical for Paraselkirkia sinica specimens from the Xiaoshiba, Chengjiang and Qingjiang Lagerstätten. Both internal features (such as the gut tract) and external features (such as the introvert, pharynx, and tube) have an underlying brownish, reddish or yellowish colouration, caused by iron oxide derived from the weathering of pyrite, which in turn was deposited on the organic tissues by the action of sulphate-reducing bacteria under anaerobic conditions. This is a reasonably common feature of Cambrian Lagerstätten, seen in deposits such as the Chengjiang of South China, and the Fezouata of Morocco.

Oocyte-bearing Paraselkirkia sinica from the Cambrian Stage 3 Xiaoshiba Lagerstätte. (a), (b) YKLP 12089: (a) incomplete specimen showing partly preserved introvert, digestive tract and oocytes; (b) close-up (see location in (a)) showing oocytes within possible tubular ovaries. (c)–(e) YKLP 12350: (c) nearly complete specimen showing partly preserved introvert; (d) interpretative drawing; (e) fluorescence image (close-up, see location in (c)) showing oocytes. (f, g) YKLP 12351: (f) incomplete specimen (introvert missing) with oocytes within the tube; (g) close-up (see location in (f)) showing the egg cluster. (h), (i) YKLP 12352: (h) complete tube showing oocytes; (i) close-up (see location in (h)) showing up to 30 eggs seemingly organised in longitudinal rows. Abbreviations: ann, annulation; ct, cuticular conical tube; dt, digestive tract; gc, gut contents; in, introvert; oo, oocytes; ph, pharynx; sc, scalid; tr, trunk. Scale bar: 1 mm (a), (c), (d), (f), and (h) and 500 μm (b), (e), (g), and (i). Yang et al. (2021).

Some of the 200 specimens collected by Yang et al. were empty tubes, but many showed the remains of soft parts. Notably, eleven had clusters of ovoid elements below the midline of the trunk, with relatively sharp rounded outline. These ovoid elements often appear as conspicuous dark spots, between 300 and 450 μm in diameter. In some cases these elements clearly lie on top of the intestine and are overprinted by the annulated pattern of the tube, which Yang et al. take to indicate that they were located within the interspace between the digestive tract and the inner body wall. These objects are not randomly scattered within the body cavity, but instead, form relatively coherent clusters, in some cases spread out in elongate clusters; in some cases these appear to be arranged on either side of the gut, though in others they are clearly only on one side.

Yang et al. believe that the consistent location, shape and size of these elements strongly point towards these being eggs carried within the body cavities of female worms. Element mapping of these clusters revealed the presence of carbon, aluminium, and silicon, as well as iron and phosphorus. Micro-computerised tomography revealed exquisite details of the scalids and digestive tract, but yielded no useful information on the clustered elements. However, when one cluster was subjected to fluorescence imaging this revealed possible external envelope, which was weekly fluorescent and had sharp external margins, and an inner core, which was more strongly fluorescent, and appeared to be detached from the outer envelope.

 

Paraselkirkia sinica from the Cambrian Stage 3 Xiaoshiba Lagerstätte: general morphology and brachiopod epibiont. (a) YKLP 12356, complete specimen showing body features and a poorly preserved Lingulate Brachiopod (arrowed) attached to the tube. (b), (e) YKLP 12354: (b) a Kutorginate Brachiopod (arrowed) attached close to the posterior opening of the tube; (e) close-up of the Brachiopod. (c), (d), (f) YKLP 12353: (c) similar attachment by a Lingulate Brachiopod (arrowed); (d) close-up of the Brachiopod; (f) micro-computed tomography image showing the attached Brachiopod. (g)–(i) YKLP 12355, specimen with complete tube and egg cluster: (g) general view; (h) close-up (see location in (g)) with the left three oocytes (arrowed); (i) micro-computed tomography image (showing poorly survived oocytes (arrowed). (j) YKLP 12085, micro-computed tomography image showing a complete, well-pyritised specimen showing detailed structures (e.g., introvert scalids, digestive tract, and the boundary between the tube wall and matrix (arrowed). Micro-computed tomography scanning resolutions for (e), (h) and (i) are 3.56 μm, 3.96 μm and 4.92 μm, respectively. Abbreviations: an, anus; ls, larval shell, others as above. Scale bars: 1 mm (a)–(c), (g), 500 μm (f), (h), (i), and 2 mm (d), (e), (j). Yang et al. (2021).

Twenty two of the two hundred specimens have a single Brachiopod, either a Lingulate or a Kutorginate, attached to one side of the posterior end of the tube. This is consistent enough to suggest there may have been some form of symbiotic association in life. As far as Yang et al. know, the only example of epibiontic symbiosis recorded from the Xiaoshiba Lagerstätte, although such associations have been documented from other Cambrian Lagerstätten.

Yang et al. interpret the clusters of rounded objects observed in Paraselkirkia as eggs within the body cavities of female individuals, based on their consistent location, shape and size, which are comparable to the positioning of eggs within extant Priapulids. There is no evidence that these structures are attached to the tissues of the body cavity, which would be expected in a parasite infection, and they are clearly different from the gut contents, which can be seen as elongated pellet-like elements. The elemental composition analysis revealed a similar composition to that seen in the brooded eggs of Cambrian Bivalved Arthropods from other locations, such as Waptia from the Burgess Shale and Chuandianella from the Chengjiang Lagerstätte. The carbon presnt probably represents underlying thin carbon patches or particles of organic origin. The innability of micro-computed tomography probably also reflects their chemical make-up, and could be indicative of a low concentration in iron oxides compared with that seen in other features of these fossils. The structures shown by fluorescence imaging appear to be an external envelope surrounding an interior core made of a different substance, possibly a yolk or nucleus.

In modern Priapulids, such as Priapulus caudatus and Maccabeus tentaculatus, the gonads (ovaries in females and testes in males) are paired structures located in the posterior part of the trunk, which is a similar position to the structures seen in Paraselkirkia. These gonads can reach large sizes, compared to the rest of the body, and in mature females often occupy a major part of the primary body cavity. In macrobenthic species fertilisation is external with males and females releasing gametes (i.e. sperm and eggs) into the water. Females typically release thousands of oocytes (unfertilised eggs) at a time, through paired urogenital ducts opening on either side of the anus. External sexual dimorphism is rare in Priapulids.

 

Female reproductive organs and oocytes in extant macrobenthic and meiobenthic Priapulid Worms. (a)-(e) Priapulus caudatus. (a) Live specimen (in sea water). (b) Dissected specimen showing paired ovaries in the posterior part of the trunk. (c), (d) Ovarian sacs bearing numerous oocytes and supporting structure. (e) Yolk spherules inside the oocyte (oocyte membrane removed). (f)–(h) Maccabeus tentaculatus. (f), (h) Specimen bearing paired oocyte clusters (seen in transmitted light, under alcohol). (g) General view showing introvert crowned with tentacles (specialised scalids). (c)–(e), (g) are scanning electron microscope images. Abbreviations: bc, primary body cavity; ca, caudal appendage; gd, gonoduct; om, oocyte membrane; os, ovarian sac; ov, ovary; soc, supporting ovarial cells; tt, tentacle; ud, urogenital duct; ys, yolk spherule; others as above. (a), (b) from the Gullmarsfjord, Sweden; (c)–(e) from the White Sea, Russia) and (f)–(h) from Cyprus. Scale bars: 1 cm (a), 2 mm (b); 100 μm (c), (d), (h) and 2 μm (e). Yang et al. (2021).

Macrobenthic (large, bottom-dwelling) Priapulids such as Priapulus caudatus and Halicryptus spinulosus typically produce thousands of oocytes with diameters of up to 60-80μm, from ovaries which consist of a large number of ovarial sacs suspended between the gonoduct and a muscular strand. Meiobenthic (small, bottom-dwelling) species, however, produce smaller numbers of oocytes from simple tubular ovaries, although these tend to be larger. For example, Meiopriapulus fijiensis typically produces eight oocytes, but these are up to 250 μm in diameter, Tubiluchus corallicola produces about 20 oocytes reaching about 80 μm in diameter, and Maccabeus tentaculatus produces eight eggs, about 100 μm in diameter.

Comparative diagrams showing female reproductive organs in extant Priapulid Worms and early Cambrian Paraselkirkia. (a) Macrobenthic Priapulid Priapulus caudatus. (b) Meiobenthic Priapulid Tubiluchus corallicola. (c) Early Cambrian Paraselkirkia sinica. (simplified reconstruction). (d) Outline of the three forms represented at the same scale (from left to right: Priapulus, Tubiluchus and Paraselkirkia). (e) Mature oocytes of the three forms at the same scale. Primary body cavity in light blue, ovaries in dark blue, oocytes in yellow, muscular tissues in light red (around pharynx), digestive tissues in light orange, cuticle in grey. Retractor muscles may be present in Paraselkirkia but are not represented. Abbreviations: rm, retractor muscle; s, solenocytes; others as above. Yang et al. (2021).

Like modern Priapulids, the Cambrian Selkirkid Paraselkirkia sinica shows no sign of external sexual dimorphism, and there is nothing to indicate that fertilisation would have been internal in this species, and it is therefore presumed likely that egg fertilisation and embryonic dervelopmet would have been external. This would imply that the structures seen in Paraselkirkia sinica, which are 300–450 μm in diameter, are non-fertilised oocytes, not developing embryos. A maximum of only about 30 eggs is seen in any individual examined, sugesting that Paraselkirkia sinica follows the small-number-of-large-eggs strategy seen in modern meiobenthic Priapulids, although it would quite clearly be considered a macrobenthc were it alive today. 

The oocyte clusters retain an cohesive appearance, suggesting that they were retained in an ovarian sac, prior to being released through the urogenital ducts. In meiobenthic Priapulids today the mature oocytes are held in place by the ovarian basal lamina, although these supporting structures such as this tend to decay rapidly after death. If similar structures were present in Paraselkirkia sinica, then they may have helped to prevent the dispersal of the oocytes throughout the body cavity, The structures revealed within the oocytes by fluorescence imaging are comparible with the nucleus of the oocytes of Priapulus caudatus, which is to say about 40% of the diameter. 

Selkirkiid Worms have been interpreted variously as burrowers that possibly lived vertically embedded in sediment, or possibly epibenthic tubicolous Worms. This makes them difficult to compare directly to modern Priapulids, which are almost exclusively active infaunal burrowers. One known modern example of a tube-dwelling Priapulid does exist, Maccabeus tentaculatus, however this species reaches a maximum size of 3 mm, and forms a flimsy cylindrical tube from agglutinated plant fragments, a structure quite different from the rigid, annulated, cuticular tube of Paraselkirkia sinica. The sedentary meiobenthic Maccabeus tentaculatus uses a crown of tentacles (which are modified scalids) to catch vagile microbenthic prey such as Copepods, its tube being essentially a camouflaging device. Paraselkirkia sinica does not appear to have been particularly sedentary; it was probably less motile than non-tubicolous forms, but had a well-developed introvert, suggesting an ability to move and possibly penetrate soft sediment. It is possible that its scalids and tube annulations were anchoring features used during locomotion and excursions into the sediment, and its tube appears to have been a primarily defensive structure.

The presence of Brachiopod epibionts living on Paraselkirkia sinica also suggests a predominantly epibenthic lifestyle. Brachipods feed by filtering food from circulating water with lophophoral cilia, making it impossible for them to feed in buried in the sediment. Assuming that Cambrian Brachiopods fed in a similar way to their modern counterparts (and there is no reason to believe otherwise), then they would have been highly unlikely to regularly form a close association with an infaunal Animal. Yang et al. therefore conclude that Paraselkirkia sinica was a semi-sedentary epibenthic Animal that occasionally explored the most superficial layers of the sediment. It is possible that external fertilisation took place within the sediment.

Assumed lifestyle and reproductive mode of Paraselkirkia. (a) Living at the water-sediment interface with a juvenile Brachiopod attached near the posterior end of the tube via a short pedicle. (b) Moving slightly below the water-sediment interface for feeding or protection (e.g. possibly during moulting). (c) External fertilisation; oocytes and spermatozoids presumably emitted within sediment. (d) In-situ development of larvae. Yang et al. (2021).

Modern Brachiopods have free-swimming larvae which settle om a range of hard substrates, including the shells of many groups of invertebrates, and metamorphose into a settled form which remains attached to the same surface for the remainder of its life. Research into the Brachiopods of the Middle Cambrian Burgess Shale strongly suggests that the Brachiopods present there had already adopted this strategy, adhering to the hard tissues of Sponges in order to survive in an environment dominated by soft muds. The Brachiopods of the Xiaoshiba Lagerstätte are about six million years older than those of the Burgess Shale, and again appear to already be preferentially attaching to hard substrates in a muddy environment. 

The Brachiopods attached to the Selkirkiid Worm tubes of the Xiaoshiba Lagerstätte are small, typically about 1 mm in diameter, suggesting these were either exceptionally small species, or early stages in the development of larger Brachiopods. Yang et al. favour the latter explanation, and suggest that larger Brachiopods are absent as Paraselkirkia sinica was actually a rather poor host, moving about and interacting with sediments in ways which made it unlikely a Brachiopod settling on its tube would survive to maturity.

It is unclear whether the relationship between Paraselkirkia sinica and the Brachiopods had any benefit to the Worm (i.e. was it true mutualism rather than simply commensalism). The preference of Brachiopods for a position close to the posterior opening of the tube is also enigmatic, with Yang et al. suggesting that such a position may have given the Brachiopod access to 'food particles' produced by the anus of the Worm.

Although fairly large for a Priapulid, Paraselkirkia sinica appears to show a mode of reproduction seen only in meiobenthic species (which seldom exceed 1 mm in length) today. Meiobenthic Priapulids invest a comparatively high amount in each offspring, ensuring each has a good chance of reaching maturity, Macrobenthic species today, however, produce a large number of offspring, but invest little in them as individuals, making it less likely that each individual will reach maturity, but more likely that some of them will. This later strategy makes more sense if the offspring have a high chance of being killed by things against which the adults cannot protect them, such as predation or physical damage from the environment. Adult Selkirkiid Worms do not appear to have a close ecological match today, but the similarity in reproductive styles with modern meiobenthic Priapulids does enable us to make some assumptions about the ecological pressures facing the species' juveniles.

The similarity between the basic organisation of the reproductive system in Paraselkirkia sinica and modern Priapulid Worms suggests that this organisation originated early in the history of the group. Possible embryonic and early post-embryonic Selkirkiid Worms have been suggested from the very base of the Cambrian. These are also quite large, comparable to the large oocytes seen in Paraselkirkia sinica, potentially suggesting that similar reproductive strategies may have originated by then.

Modern Animals show a very wide range of reproductive modes, which may have appeared independently in different groups Ecdysozoans are the largest single subdivision of the Metazoa today and were already an important and diverse group in the Early Cambrian. The discovery of a reproductive  system in Paraselkirkia sinica  brings the number of Ecdysozoans from the Early Palaeozoic where we know something about their reproductive systems to three. In Early Cambrian Waptid Arthropods egg clusters have been found carried symmetrically on either side of the female's body, which suggests paired gonads as in Paraselkirkia sinica. Copulatory organs have been preserved in Ostracods from the Early Silurian Herefordshire Lagerstätte, which suggest that these Arthropods had developed internal fertilisation by this stage. 

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Lingulate Brachiopods from the Lower Devonian of the Spanish Central Pyrenees.

Lochkovian and Pragian strata from selected sections in the Spanish Central Pyrenees have provided one of the best Conodont sequences in the world permitting the development and global correlation of hi-resolution bioand chronostratigraphic scales. This scale is better for the Lochkovian, especially for the middle Lochkovian, but Pragian records in combination with records from the Barrandian area have enabled the proposal of an alternative Pragian Conodont biozonation for European sequences. Other fossils are not as abundant and have not received much attention, except the Ostracods from the Gerri de la Sal sections and some Fish remains from the Compte-I section.

Lingulate Brachiopod remains are present in many Conodont samples from Lochkovian and Pragian rocks but commonly they are in low numbers, poorly preserved and with spotty records that difficult a thorough study on this group. However, the section Compte-I presents an excellent record that enables the first systematic study of this group in the Pyrenees and its precise stratigraphical location within a detailed time framework constructed by the accompanying Conodont taxa.

In a paper published in the journal Bulletin of Geosciences on 16 May 2020, Michal Mergl of the Centre of Biology and Geosciences at the University of West Bohemia, and José Valenzuela-Ríos of the Department of Botany and Geology at the University of Valencia, present a systematic study of Lingulate Brachiopods in the Pyrenean section Compte-I, and place them into the fine biostratigraphic Conodont scale for future correlations.

The current knowledge of Lower Devonian micromorphic Lingulate Brachiopods outside Bohemia and Australia is poor and therefore not adequate for correlation and evolutionary suggestion, and any new stratigraphical exactly placed data are of high importance.

The Compte section is located near the former national road N-260 between the localities of La Pobla de Segur The Compte section is located near the former national road N-260 between the localities of La Pobla de Segur in Spanish Pyrenees. All the fossils come from the Lochkovian and lower Pragian strata of the section Compte-I. This section spans the Lochkovian to Emsian, but the betterstudied part is the Lochkovian and early Pragian. Previous studies have described in detail this part of the section and presented the first detailed bioand chronostratigraphic interpretations, and identified the three-fold Lochkovian subdivision (lower, middle and upper) and several zones within these parts. Moreover, the Lochkovian/Pragian interval was narrowed for the first time in the Pyrenean sections. Subsequently, José Valenzuela Ríos and Jau-Chyn Liao presented the first general study on thin section and microfacies to characterise the lithofacies present in the Compte-I section in 2012. A more recent study established a Conodont-based detailed correlation between the Spanish Central Pyrenees and the Prague Synform for middle and upper Lochkovian strata, including the Lochkovian/Pragian transition in the Compte-I section.

The Organophosphatic Brachiopod described by Mergl and Valenzuela-Ríos were obtained as the side product of the research primarily focused on Conodonts. In total, 140 limestone samples were collected and dissolved in diluted formic acid (6–8%) in order to search for conodonts. The insoluble residue was decanted and sieved of 1.5 mm permitted size separation of larger fragments. The finer residue (smaller than 1.5 mm) was dried and Brachiopod shells were hand-picked under the binocular lens.

 

Geographical location of studied section (A) and (B) and stratigraphical log (C) with distribution of key Conodont taxa and recorded ranges of the Brachiopods studied herein referred to the Conodont zones (right column). Abbreviations: SP, Spain; CP-I, Section Compte-I; Prag., Pragian; wo, woschmidti Zone; om-trig, omoalpha-trigonicus Zone; the base of this Zone marks the base of the middle Lochkovian; trig.-kut., trigonicus-kutscheri Zone; kut-β, kutscheri-pandora beta Zone; β-gilberti,  pandora beta-gilberti Zone; the base of this zone coincides with the base of the upper Lochkovian; gilb.-irr., gilberti-irregularis Zone; the end of this Zone marks the base of the Pragian; Icr., Icriodus; Acod., Acodina; a and ang., angus-toides; Anc., Ancyrodelloides; L, Lanea; M, Masaraella; Ped., Pedavis. Conodont records bracket the base of the Pragian between beds 49–55. Mergl & Valenzuela-Ríos (2020).

The first specimen described are assigned to the genus, Barrandeoglossa (due to the fragmentary nature of the specimens, none are assigned to species level). This is a small fragment of dorsal valve which shows convex smooth larval shell. That is posteriorly bordered by a thickened posterior shell margin forming distinct convex strip tapering toward the apex. Other details are unknown.

Although the material is very poor, the thickened posterior margin is a distinct feature of the genus. The fragment may belong to Barrandeoglossa perneri, a common species in the Kotýs Limestone of the Lochkovian age in the Barrandian area of the Czech Republic, but because data about the ventral valve are unknown, an open taxonomical position is preferred.

 

(A), (D), (E), (I), (J), (V), (W), (X) Acrosaccus sp. B; (A), (I) incomplete ventral valve exterior, ventral and posterolateral views, sample CP-I/40e, specimen MGUV-36184; (D), (J) incomplete ventral valve exterior, ventral and posterolateral views, sample CP-I/40A, specimen MGUV-36185; (E) incomplete ventral valve exterior, ventral view, sample CP-I/40A, specimen MGUV-36186; (W) incomplete dorsal valve exterior, oblique view, sample CP-IW/52C, specimen MGUV-36187; (V), (X) incomplete dorsal valve exterior, oblique and dorsal views, sample CP-IW/52C, specimen MGUV-36188. (B), (C), (G) Schizotreta sp. B; (B), (C) incomplete dorsal valve exterior, dorsal and posterolateral views, sample CP-I/48, specimen MGUV-36189; (G) incomplete dorsal valve exterior, dorsal view, sample CP-I/40A, specimen MGUV-36190. (F), (H), (K) ?Praeoehlertella sp.; incomplete dorsal valve exterior, dorsal and posterolateral views and detail of microornament (F), sample CP-I/34, specimen MGUV-36191. (L)–(S) Chynithele aff. fritschi; (L), (O), (Q) dorsal valve exterior, oblique and dorsal views, and detail of rugellate ornamentation, sample CP-IW/51top, specimen MGUV-36192; (M), (N), (P), (R), (S) dorsal valve exterior, oblique and dorsal views, details of microornament (P), (S), and detail of rugellate ornamentation (R), sample CP-IW/51B, specimen MGUV-36193. (T) Barrandeoglossa sp.; fragment of dorsal valve, dorsal view, sample CP-I/34, specimen MGUV-36194; (U) Lochkothele cf. intermedia, incomplete ventral valve exterior, ventral view, sample CP-I/30F, specimen MGUV-36195. (Y), (Z) Schizotreta sp. A, dorsal valve exterior, oblique and dorsal views, sample CP-I/34, specimen MGUV-36196.  Length of bars is 1 mm (A), (B), (V), 500 μm (D), (E), (G), (J), (L)–(O), (V)–(Z), 200 μm (C), (H), (I), (K), (T), 100 μm (Q), (R), 20 μm (P), (S), and 10 μm (F). Mergl & Valenzuela-Ríos (2020).

The next specimens are assigned to the genus Acrosaccus, as Acrosaccus sp. A. These are a hundred dorsal valves and their fragments. Shell moderately thick-walled, 6 mm wide in the largest known fragment. The dorsal valves range from elongate elliptical to gently transversally oval outlines, with some valves nearly circular. Anterior and lateral margins are evenly rounded, the posterior margin is slightly less rounded. Valve convexity is low both in transverse and longitudinal profiles, but the brephic valve is prominently convex and elevated above the adjacent surface of mature shell. The apex is situated in posterior ¼ of the valve. The larval shell subcircular, evenly arched and smooth, 350–400 μm wide. It is bounded by an elevated periphery from the post-larval shell. The brephic shell is gently transverse, about 1.5 mm broad, with transversally oval outline and evenly rounded periphery. Its surface is covered by weak growth fila more distinct around the periphery. Generally smoother surface distinguishes the brephic shell from the mature shell surface. Whole brephic shell with the larval shell attached to its posterior is gently inclined and posterodorsally faced.

The ornament of the dorsal valve consists of entire or imperfect concentric rugellae. New imperfect rugellae originate lateral to the apex, at about one-fourth of shell length. Some rugellae are short, and rapidly change into low, rope-like fila on surface of the interspace. Entire rugellae rapidly increase in size with shell growth, with new rugella always somewhat higher than the preceding one. Interspaces vary in width, having a flat bottom or bearing low, rope-like concentric fila. Rugellae are lamellose, resting almost perpendicularly to shell surface, with weakly thickened bases and gently thicken crests. Rugellae between the apex and the posterior margin are lower but the interspaces have coarser growth fila. Surface of interspaces and slopes of rugellae are covered by oblique lines. Microornament consists of densely crowded uniformly sized subcircular pits of 3 μm diameter in regular honeycomb-like arrangement.

Interior of the dorsal valve bears large, obliquely  elongate and clearly impressed central muscle scars inclined 50° each another and separated each another by a low median ridge. Visceral area is elongate rhomboidal in outline, 30% as wide as the valve and anteriorly extends to midvalve. Its posterior part is divided by a thin myophragm. Anterior and lateral margins bear distinct imprints of radially arranged terminal canals of the vascular system. Some of canals extend from gently divergent narrow canals of vascula media, which extend from anterior corners of central muscle scars. Small concave epithelian cell moulds cover the visceral field and adjacent parts of he interior. Ventral valve is unknown.

The fragments are similar to some Acrosaccus-like discinids with prominent concentric rugellae. However, lack of information about the ventral valve makes generic affiliation of Spanish specimens tentative. Several similar discinids of the Acrosaccus shape of the Lower Devonian age are known from the Barrandian area of the Czech Republic but none of them has surface of interspaces and slopes of rugellae covered by oblique fila as exhibits the Acrosaccus sp. from Pyrenees. Likewise, none of fragmental ventral valves from our sample from Compte-I section show this distinct microornament. Among the European species, Acrosaccus sp. from the Suchomasty Limestone (Emsian) is most similar, but lacks the above mentioned oblique fila and its rugellae are much robust and much closely spaced. A similarly shaped Acrosaccus vertex was described from the Chýnice Limestone (upper Emsian), but the lamellose rugellae of Acrosaccus sp. are distinctly higher and more closely crowded. Acrosaccus sp. from the Acanthopyge Limestone (Eifelian) of the Barrandian area has similar lamellose rugellae in the dorsal valve, but its rugellae are lower than rugellae of Acrosaccus sp. Similar species ?Acrosaccus sp. has been described from several limestone members of the Lower Devonian age of New South Wales, Australia, but none of illustrated specimens has as high rugellae as Acrosaccus sp. from Pyrenees. Unfortunately, the fine details is Australian shells are unknown.

The ventral valve of Acrosaccus sp. is unknown from our samples, while the dorsal valves are numerous and fairly well preserved. This disproportion may be explained by disintegration of thinner and much fragile ventral valve, likely having low conical shape. Similar disproportion in number of dorsal and ventral valves has frequently been observed in samples in the Barrandian area. Similar variations of the shell outline observed in Acrosaccus sp. are known in other Lower Palaeozoic Discinoids (e.g. Orbiculoidea sp.). It is likely related to biotope of a particular specimen. It is worth a mention that the outline of brephic shell is constantly circular despite differently outlined postlarval shells. Pyrenean specimens surely belong to a new species but shortage of important piece of regarding the ventral valve led Mergl and Valenzuela-Ríos to prefer an open taxonomic position.

This species is the commonest Lingulate Brachiopod in samples of the Compte-I section.

The next group of specimens are also referred to the genus Acrosaccus, as Acrosaccus sp. B. This material comprises ten fragmentary ventral and dorsal valves.

The available shells are imperfect and therefore the species is poorly known. Fragments indicate that shell is thin-walled and subcircular to slightly elongate oval in outline. Dorsal valve has prominent, moderate convex subcircular brephic shell which is strongly tilted posterodorsally above a steep posterior slope of valve. Size of the brephic valve is around 500 μm. The mature shell bears fine thread-like concentric growth fila.

The ventral valve is low conical, with distinct brephic valve which slightly overhang the earliest part of the pedicle track. Pedicle track is narrowly triangular, with posteriorly moderate expanding listrial plates and narrow evenly broad inner listrial plate. Length of the pedicle track is about 1.5 mm. It is posteriorly closed by mature shell bearing the same external ornament as adjacent posterolateral parts of the valve. The mature ventral valve is covered by concentric growth fila gradually increasing in a size toward shell periphery. The early lines encircling the apex are thread like and very fine, but fila from 2 mm sized shell are much higher, convex, with broad bases separated by unevenly sized interspaces.

Attribution of ventral valves to dorsal valves is somewhat tentative, but it is based on similar type of ornament and biconvex shells profile similar to some species of the genus. Acrosaccus sp. A differs by distinct type of microornament and by coarse rugellae. This species is present in samples CP-I/34, CP-I/40A, CP-I/46B, CP-I/51B, CP-I/56, CP-IW/52C of the Compte-I section.

 

(A), (D), (E), (I), (J), (V), (W), (X) Acrosaccus sp. B; (A), (I) incomplete ventral valve exterior, ventral and posterolateral views, sample CP-I/40e, specimen MGUV-36184; (D), (J) incomplete ventral valve exterior, ventral and posterolateral views, sample CP-I/40A, specimen MGUV-36185; (E) incomplete ventral valve exterior, ventral view, sample CP-I/40A, specimen MGUV-36186; (W) incomplete dorsal valve exterior, oblique view, sample CP-IW/52C, specimen MGUV-36187; V, X, incomplete dorsal valve exterior, oblique and dorsal views, sample CP-IW/52C, specimen MGUV-36188. (B), (C), (G) Schizotreta sp. B; (B), (C) incomplete dorsal valve exterior, dorsal and posterolateral views, sample CP-I/48, specimen MGUV-36189; (G) incomplete dorsal valve exterior, dorsal view, sample CP-I/40A, specimen MGUV-36190. (F), (H), (K) ?Praeoehlertella sp.; incomplete dorsal valve exterior, dorsal and posterolateral views and detail of microornament (F), sample CP-I/34, specimen MGUV-36191. (L)–(S) Chynithele aff. fritschi; (L), (O), (Q) dorsal valve exterior, oblique and dorsal views, and detail of rugellate ornamentation, sample CP-IW/51top, specimen MGUV-36192; (M), (N), (P), (R), (S) dorsal valve exterior, oblique and dorsal views, details of microornament (P), (S), and detail of rugellate ornamentation (R), sample CP-IW/51B, specimen MGUV-36193. (T) Barrandeoglossa sp.; fragment of dorsal valve, dorsal view, sample CP-I/34, specimen MGUV-36194; (U) Lochkothele cf. intermedia, incomplete ventral valve exterior, ventral view, sample CP-I/30F, specimen MGUV-36195. (Y), (Z) Schizotreta sp. A, dorsal valve exterior, oblique and dorsal views, sample CP-I/34, specimen MGUV-36196. Length of bars is 1 mm (A), (B), (V), 500 μm (D), (E), (G), (J), (L)–(O), (V)–(Z), 200 μm (C), (H), (I), (K), (T), 100 μm (Q), (R), 20 μm (P), (S), and 10 μm (F). Mergl and Valenzuela-Ríos (2020).

The next specimens are referred to the genus Chynithele as Chynithele aff. fritschi. This material comprises two dorsal valves. 

The dorsal valve is subcircular, about 2 mm wide, gently concave, thick-walled, rectimarginate. The apex is situated at posterior one-fifth of the valve. The larval shell and brephic shell are not clearly differentiated. The brephic shell is weakly convex, about 280 μm wide, encircled by very fine entire concentric rugellae of the early mature shell.

Ornamentation consists of mostly entire rugellae, with rare inserted imperfect rugellae which rapidly attain the same size of nearby entire rugellae. Sizes of rugellae progressively increase with shell growth. Rugellae are gently inclined forward. The crest of each rugella is broader than the base of rugella and form a short shelf that slightly overhang the anterior slope of the rugella. Interspaces have flat or shallow concave bottom and are slightly wider than crests of adjacent rugellae. Microornament consists of small circular pits of 3 μm diameter closely spaced. Pits covers entire surface of the mature shell.

Both specimens show distinct features of the genus: a weakly concave dorsal valve, generally entire, prominent and gradually larger rugellae with overhanging crests, and a microornament of fine circular pits. The genus Chynithele is represented by several successive species (Chynithele fritschi, Chynithele ventricona, Chynithele intermedia, Chynithele amoena) in the Barrandian area. These are known from the Emsian to Eifelian. Apart from the stratigraphically earliest species, others species differs from the Spanish specimen by generally lower much robust rugellae and much transverse outline of dorsal valves. The species Chynithele fritschi from the lower Emsian (Zlíchov Formation) was erected on poorly preserved dorsal valve showing only internal characters. This species comes from the upper part of the Zlíchov Formation (lower Emsian) and its relationship to approximately coeval Chynithele ventricona requires a detail revision. Undescribed species of Chynithele that differs from the Emsian and Eifelian species of the Barrandian area is also known from the Kotýs Limestone (Lochkovian) of the Barrandian: this to date unnamed species has similar shaped concentric rugellae as the Spanish species.

The next specimen, one fragment of ventral valve, is assigned to the genus Lochkothele, as Lochkothele cf. intermedia.

The apical part of the ventral valve belongs, judging from growth lines, to subcircular, gently elongate thin walled shell. Larval shell is slightly transversely oval, about 500 μm wide, with poorly defined border. The pedicle track is deeply incised, 0.8 mm long, evenly wide along its whole length. The track is posteriorly terminated by reversely V-shaped incision. The floor of the pedicle track is undifferentiated, evenly concave and covered by growth lines. Exterior of mature ventral valve is covered by growth fila accentuated in regular intervals into low rugellae. The first rugella appears 0.7 mm posteriorly from the apex and other rugellae are next arranged in regular distances 0.2 mm apart. Microornament is unknown in the specimen from Compte-I section.

The earliest Lochkothele is known from the Wenlock (Homerian). Several, usually rare occurrences of Lochkothele come from the Lochkovian, the Emsian and the Eifelian. All known species of the genus have distinct semitubular undifferentiated pedicle tracks, weakly ornamented ventral valves with closely packed concentric growth fila, and thin and low thread-like rugellae. The single available fragment may belong to Lochkothele intermedia, the common species of Lochkovian age in the Barrandian area.

The next specimen, a fragment of dorsal valve, is assigned to the genus Schizotreta, as Schizotreta sp. A.

Single available dorsal valve is elongate elliptical with the marginal apex and belongs to individual approximately 3 mm long judging from shaping of rugellae. The shell wall is moderately thin. The valve is weakly convex in transversely profile and gently depressed in the median sector. The larval shell is distinct, smooth, subcircular, 300 μm wide, convex and gently inclined towards the posterior margin. The postlarval shell is covered by rod-like concentric rugellae with new ones originating in a posterolateral sector by intercalation. Both concentric and new rugellae are uniformly sized, arranged in regular intervals 120–150 μm apart along the axis of valve. Rugellae are gently inclined outwards, with anterior slope shortly undercut. Broad and flat floor of interspaces bears growth fila, which are distinct posterolaterally and suppressed anteromedially.

The valve is similar to dorsal valves of Schizotreta vaneki from the Chýnice Limestone of the Barrandian area. However, this species is significantly younger (upper Emsian) and the ventral valve is unknown in the Compte-I locality. Moreover, the Spanish specimen differs by more elongate outline which has weak taxonomic significance and may be exaggerated by a tectonic deformation. Therefore Mergl and Valenzuela-Ríos prefer an open taxonomical position.

The next group of specimens, thirty-one fragments of dorsal valves, are also assigned to the genus Schizotreta, as Schizotreta sp. B.

The dorsal valve is elongate oval, 2 mm wide, with maximum width at posterior one-third. Anterior margin is semicircular, sides are less rounded and the posterior margin is only gently curved, with nearly straight part posterior to the brephic valve. The shell convexity is very low and decreases anteriorly. The apex is situated to one-fourth to one fifth of the valve, separated from the posterior margin by narrow band of steeply sloping shell bearing some rugellae. Larval and brephic shells are poorly known due preservation. The mature shell is covered by distinct but low, rope-like concentric rugellae, which are uniformly sized. Size of particular rugella gently changes with its location on valve; posterolateral part of rugella is higher and more distinct than its anteromedian partitions. Interspaces are broader than rugellae, with flat floor and surface covered by uneven growth fila. Interior and ventral valve are unknown.

The valve differs from Schizotreta sp. A from the Pyrenees and from Schizotreta vaneki from the Chýnice Limestone (upper Emsian) of the Barrandian area by much densely crowded concentric rugellae.

The next specimen, an incomplete dorsal valve, is referred to the genus Praeoehlertella, as ?Praeoehlertella sp.

The dorsal valve is poorly known, but from the development of growth lines may be deduced that shell is very broadly oval to almost circular, with evenly curved margins including the posterior margin. The apex is situated at posterior one-third, with low posterior slope bearing gently coarser ornament than developed on the rest of the valve. The brephic shell is about 300 μm wide, gently vaulted, smooth and slightly tilted posterodorsally. The surface of post-larval shell bears densely crowded, low and broad concentric growth fila separated by narrow slit-like interspaces. One distinct largely extending growth lamella is present. Microornament consists of minute circular pits (about 1 μm in diameter) densely covering entire surface excluding the brephic shell. 

Mergl and Valenzuela-Ríos's material is poor and therefore is referred to Praeohlertella with uncertainty. Outline, convexity, location of the apex on the dorsal valve, and type of ornamentation are close to Praeohlertella lukesi from the Suchomasty Limestone (upper Emsian) of the Barrandian area.

The final group of specimens, 44 dorsal and 11 ventral valves, are referred to the Acrotretid genus Havlicekion as Havlicekion cf. holynensis.

The shell of these specimens is fairly large for the genus with the ventral valve 1.5 mm tall having a broadly unisulcate commissure. The dorsal valve is subcircular, with evenly rounded posterior margin and weak and broad sulcus. Larval shell has two distinct nodes. The larval shell is 200 μm wide, slightly overhanging the posterior edge of post-larval shell. Dorsal pseudointerarea is anacline, with straight anterior edge and very shallow widely triangular median groove. Median septum is high, thin, having steeply sloping anterior edge towards a weakly defined marginal brim. It occupies 80% of valve length. The surmounting plate is inclined at about 45° toward shell floor, the lower rod is at about 35°. The surmounting plate anterior tip is at the midvalve, the tip of the lower rod at two-third of the valve. The crest of surmounting plate is rounded, slightly flattened near its anterior end. The end is blunt. The proximal part of the lower rod is less distinct, but its distal part is distinct but less robust than the surmounting plate. Anterior end of the lower rod deviates to right or left side from the plane of symmetry. Cardinal muscle scars are large, elongate oval, 30% as long as the valve.

The ventral valve is tall cone, with clearly flattened and fairly broad catacline ventral pseudointerarea. The larval shell is asymmetrically conical with slightly extended part near the foramen. The larval shell is pierced by large circular ventrally directed foramen just above steep posterior slope. Lateral and anterior slopes of the valve are gently convex in lateral profile having gradually changing slope from moderate near beak to steeper in later growth stages.

Ornament of the dorsal valve consists of low concentric rugellae of uniform size separated by narrow interspaces. The concentric ornament is more distinct on dorsal valve. The ventral valve is weakly rugellate having rugellae crossed by radial striae. The width of rugellae is 8 to 10 μm. This combination forms a drapery-like relief, which obscure the rugellate ornament. Larval shell is covered by uniformly sized flat-bottomed circular pits of 5 μm diameter. Pits are regularly arranged, closely spaced and only rarely intersecting.

The specimens are very similar to Havlicekion holynensis from the Kotýs Limestone of the Lochkovian age of the Barrandian area. There is only one difference on dorsal median septum. Distance between the surmounting plate and lower rod is smaller in Havlicekion holynensis and a blade-like part of septum between them is distinctly shorter than those in Havlicekion cf. holynensis from Spain. The surmounting plate on Havlicekion holynensis may be weakly concave in lateral profile and is triangular in a cross-section. but this is straight and rod-like in Havlicekion cf. holynensis . Current knowledge of phenotypic plasticity of the shell interior of Biernatiids is not satisfactory for evaluation of the taxonomical value of these minor differences. Therefore Mergl and Valenzuela-Ríos are aware using observed minor differences between specimens for definition of separate species.

 

Havlicekion cf. holynensis; (A)–(C), (S), (U), (W) ventral valve exterior, oblique, lateral and anteroventral views, side view to larval shell (S), contact between larval and mature shell (U) and ornament of mature shell (W), sample CP-IW/52C, specimen MGUV-36197; (D) ventral valve broken wall, posterolateral view, CP-IW/52C, specimen MGUV-36198; (E), (H), (T), (V) ventral valve exterior, posterior view, detail of larval shell in posterior and lateral views, and pitted ornamentation of the larval shell, sample CP-IW/52C, specimen MGUV-36199; (F) dorsal valve exterior, dorsal view, sample CP-I/12, specimen MGUV-36200; (G), (J) dorsal valve exterior, dorsal and oblique views, sample CP-I/40A, specimen MGUV-36201; (I) ventral valve exterior showing larval shell and ornamentation of ventral pseudointerarea, sample CP-I/12, specimen MGUV-36201; (K) dorsal valve exterior, dorsal view, sample CP-I/12, specimen MGUV-36202; (L) dorsal valve interior, ventral view, sample CP-IW/52C, specimen MGUV-36203; (M) dorsal valve interior, ventral view, sample CP-IW/52C, specimen MGUV-36204; (N) dorsal valve interior, ventral view, sample CP-I/12, specimen MGUV-36205; (O)–(Q) dorsal valve interior, ventral, side and oblique views, sample CP-I/40A, specimen MGUV-36206; (R) dorsal valve interior, side view, sample CP-IW/52C, specimen MGUV-36207. Length of bars is 200 μm (A)–(G), (J)–(R), 100 μm (I), 50 μm (H), (S), (T), (W), and 20 μm (U), (V). Mergl & Valenzuela-Ríos (2020).

Our current state of knowledge of the Silurian and Devonian organophosphatic micromorphic Brachiopods extracted from carbonates is rather weak. Many issues of their palaeogeography, palaeoecology, evolutionary history and some aspects of their shell morphology remain to be solved. This deficiency contrasts with extent of information about morphology and evolutionary history of the group based on samples from early Cambrian to latest Ordovician; mostly extensive or summarizing works have been published in last decades.

This difference is not caused by sampling methods, because organophosphatic microbrachiopods can be easily extracted by weak acid solution from limestone of any age. However, unlike Cambrian and Ordovician rocks, abundance, diversity and disparity of organophosphatic Microbrachiopods are markedly lower in the Silurian and Devonian carbonates. Therefore, due to their rarity and unattractiveness, few works are focused on Devonian Microbrachiopods. To date, the reliable data referred to them came from two distant areas: New South Wales, Australia and the Barrandian area of the Czech Republic. Scattered data about the ubiquitous Biernatid Opsiconidion also comes from other areas with Devonian rocks (Arctic Canada, Novaya Zemlja, Mongolia, Podolia, Ukraine, Carnic Alps). Research on these Devonian Microbrachiopods has often been focused on shell morphology and ultrastructure of larval shell.

Regarding current knowledge of Devonian Microbrachiopods, the new Pyrenean record of Microbrachiopods from lower Lochkovian to early Pragian represent important contribution to elucidate the Lower Devonian Microbrachiopod history. Three taxa ascribed as Acrosaccus sp. A, Schizotreta sp. B and Havlicekion cf. holynensis range across the Lochkovian and reach the stratigraphic level very close to the Lochkovian–Pragian boundary. Barrandeoglossa sp., Schizotreta sp. A, ?Praeoehlertella sp., and Lochkothele cf. intermedia are restricted to middle Lochkovian in the section Compte-I , and only Acrosaccus sp. B crosses the Lochkovian–Pragian boundary. The Pyrenean records of Chynithele aff. fritschi begin close to the base of the Pragian in Compte-I  section. Although fragmentation and rarity of shells hamper accurate taxonomic evaluation, a general composition of lingulate fauna indicates uniformity of Microbrachiopods in Early to Middle Devonian tropical marine fauna.

The Microbrachiopod assemblage from Spanish Central Pyrenees is similar to ones known from the Lochkovian and the Pragian of the Barrandian area, the Czech Republic. Dominance of small Discinoids, a Glossellid Barrandeoglossa and a Biernatiid Havlicekion cf. holynensis indicates a moderately shallow environment, similar to depositional environment of the Kotýs Limestone (Lochkov Formation, Lochkov) in the Barrandian area. This agrees with previous palaeoenvironmental interpretations. Absence of a Lingulate Paterula and presumable absence of Biernatiid Opsiconidion (although cannot be excluded, that small fragments of Biernatiids from Pyrenees belong to this genus) indicate a limited influx of deeper and/or hypoxic environment in Pyrenees. Large thick-shelled Lingulates are totally absent in our samples from Pyrenees. In general, the Spanish Pyrenees assemblage is remarkably similar to the ones from Lower Devonian, Barrandian area and to ones reported from New South Wales, Australia. The Spanish Pyrenean fauna likely represents one of so far rare examples of the 'standard' Microbrachiopod association of shallow to moderately deep environment of Lower Devonian tropical climatic zone.

All three Devonian occurrences of Microbrachiopods (New South Wales, Central Bohemia, and newly the Spanish Pyrenees) are typical by dominance of very small sized ('dwarf') shells. All areas were located within tropical climatic zone with intensive carbonate production on reefs and carbonate platforms in the Early Devonian. Small size of organophosphatic Brachiopods in these areas greatly contrasts with many times larger shells of the organophosphatic Brachiopods that populated siliciclastic shelves, mostly but not exclusively present in high-latitude sites during the Early Devonian. More or less well known medium- to large-sized Discinoids are known from Argentina, Bolivia, Uruguay, Paraguay and Brazil, Antarctica, Falkland Islands, South Africa, and Libya and Algeria. Similarly large Discinoids are known from siliciclastics of the Rhenish Massif, Germany and New York, U.S.A.. Some represents the largest organophosphatic Brachiopods so far known with shell size exceeding 60 mm width. Indeed, their shell size indicates that phosphorus was easily accessible in surrounding water.

 

Palaeogeographic reconstruction for the Early to Middle Devonian time showing geographical distribution of Biernatiids (asterisk) and large Discinoids (circles) (for references see text). The sites on the reverse Earth hemisphere are marked outside map. Mergl & Valenzuela-Ríos (2020).

Small size of organophosphatic Brachiopods from limestone was likely consequence of nutrient scarcity, especially phosphate depleted waters at ancient reefs. On the Recent reef complexes nutrient-poor (oligotrophic) waters dominate. That is in striking contrast with enormous biotic diversity at the Recent reefs. The similar circumstances could be suggested for the Devonian reefs. The dissolved inorganic phosphorus in form of phosphate is particularly important for reef organisms in the Recent reefs. Reef waters have about only 20% concentration of phosphate in comparison with concentration of dissolved phosphate in deep waters. In areas with upwelling waters the input of phosphate may be significantly higher. Run-off of nutrient rich water from land is another significant input of phosphate into nearby sea.

Run-off of phosphate from land to shallow siliciclastic shelves was likely the significant trigger for production of large shelled organophosphatic Brachiopods in the Devonian. This could be the case of large-sized Discinoid populations on high latitude cratonic basins on Gondwana or low latitude shelves with significant input of siliciclastics. Recent large Discinoid Brachiopods occur in shallow littoral zone near upwelling sites (west coast of Africa and west coast of South America) while their tropical allies are small sized, rare and often occupying a cryptic habitat. Similarly, the analogous depletion of phosphate in waters in the Devonian reef contributed miniaturization and retreat of organophosphatic brachiopods from this environment in mid-Palaeozoic.

The Microbrachiopod assemblage from Spanish Central Pyrenees is similar to ones known from the Lochkovian and the Pragian of the Barrandian area, the Czech Republic. Hence the Spanish Pyrenean fauna likely represents one of so far rare examples of the 'standard' Microbrachiopod association of shallow to moderately deep environment of Lower Devonian tropical climatic zone. Small size of organophosphatic Brachiopods from limestone was likely consequence of nutrient scarcity, especially phosphate depleted waters around the ancient reefs. This also contributed to retreat of organophosphatic Brachiopods from this environment during the mid-Palaeozoic.

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Brachiopod communities of the Early Cambrian Guanshan Lagerstätte of Yunnan Province, China, and their associated facies.

Discoveries of spectacular soft-bodied animal assemblages from Cambrian Konservat-Lagerstätten around the world have provided incredible insights into the anatomy, behaviour, ecology and early evolution of complex Metazoans. Early Cambrian Konservat-Lagerstätten from China, such as the Niutitang Fauna, Chengjiang Biota, Guanshan Biota, Shipai Biota, Balang Fauna, Kaili Biota and the newly discovered Qingjiang Biota, span a wide range of geological time and provide a unique opportunity to map changes in Early Cambrian ecological communities over time. The Guanshan Biota (Cambrian Series 2, Stage 4), one of the oldest Konservat-Lagerstätten from South China, occurs in the Wulongqing Formation in eastern Yunnan. Younger than the famous Chengjiang and Malong biotas (Cambrian Series 2, Stage 3), but older than the Kaili and Burgess Shale biotas (Miaolingian Series, Wuliuan Stage), the Guanshan Biota is a significant evolutionary bridge in our understanding of the chronology of the Cambrian radiation and its aftermath. Recent intensive, although preliminary, excavations reveal that the Guanshan Biota is composed of 14 major animal groups and various ichnotaxa. Uniquely, the Guanshan Biota is dominated by Brachiopods, which serves to distinguish it from all other Cambrian Konservat-Lagerstätten, which are dominated (in terms of diversity and relative abundance) by Euarthropod groups. Faunal overturn between the Chengjiang, Malong and Guanshan biotas suggests that the sessile benthic members of the assemblages are affected by the same factors that affect mobile Trilobites. Furthermore, the Wulongqing Formation is characterized by bioturbated, thinly bedded sandstones, siltstones and mudstones, which crop out widely in eastern Yunnan, South China and represent a transgressive systems tract directly after the Hongjingshao Formation. Previous, very generalised, sedimentological work on the Wulongqing Formation suggests a relative shallow (shoreface to offshore transitional) depositional environment, which is distinct from the generally deeper water (in some cases slope to basin) setting of most other early Cambrian deposits that preserve soft tissues.

Continuous exploration and research in the Guanshan Biota has led to the discovery of multiple new localities and increased systematic descriptions of the fossil taxa, including documentation of one of the oldest examples of kleptoparasitism in the fossil record. The Wulongqing Formation is generally poorly exposed at most sites and artificial cover by urban landscaping has obscured many of the classic flat-lying sites. There has been a dearth of even basic ecological analyses of the faunal assemblages from the Guanshan Biota, and the detailed sedimentology and lithology of the succession are very poorly resolved.

In a paper published in the Journal of the Geological Society on 18 September 2020, Feiyang Chen, Glenn Brock, and Zhiliang Zhang of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, and the Department of Biological Sciences at Macquarie University, Brittany Laing, also of the Department of Biological Sciences at Macquarie University, and of the Department of Geological Sciences at the University of Saskatchewan, and Xinyi Ren and Zhifei Zhang, also of the State Key Laboratory of Continental Dynamics, Shaanxi Key Laboratory of Early Life & Environments and Department of Geology at Northwest University, aim to comprehensively document the lithofacies and sedimentology of the basal part of the Wulongqing Formation hosting soft-bodied fossils at the Shijiangjun section, the best-exposed succession in Wuding county, eastern Yunnan. Zhang et al. hope these data will help to decipher the relationships between microfacies, sedimentary events and faunal overturn after transgression and how fluctuations in depositional environments affect the faunal composition during the later stages of the Cambrian evolutionary radiation.

 

Localities of the Guanshan Biota and distribution of lower Cambrian outcrops in eastern Yunnan, South China. The Shijiangjun section in Wuding county is represented by locality 1. Chen et al. (2020).

The uppermost Hongjingshao and lower Wulongqing formations are exposed in the newsectionwith a very clear conformable stratigraphic contact. This provides an opportunity to document temporal changes in the faunal composition and sedimentary environments at the centimetre scale based on lithological, sedimentological, palaeontological and ichnological evidence. Chen et al.'s detailed study enables an interpretation of the depositional environment associated with the lower Wulongqing Formation and facilitates a better resolution of the process and drivers of faunal overturn that distinguish the Guanshan faunas from the Wuding, Malong and Kunming areas.

 

Stratigraphic column, sedimentology, facies, structures, bioturbation index and pie charts of relative faunal abundance in the lower Wulongqing Formation at the Shijiangjun section. The 41 rock samples taken for cutting and polishing are marked on the left-hand side of the column. The datum point (0 cm) is the boundary between the upper Hongjingshao and lower Wulongqing formations. Four facies (F1, F2, F3 and F4) were recognized. The bioturbation index for each polished rock samples was evaluated based on the extent to which bioturbation disrupted the primary bedding. The composition of the fossil assemblage is shown by pie charts at the phylum level and the brachiopod genus level. Abbreviations: Ass., assemblages; BI, bioturbation index; F., facies. HJS, Hongjingshao Formation; Sam., sample number; WLQ, Wulongqing Formation. Chen et al. (2020).

The Shijiangjun section was measured through the uppermost Hongjingshao and lower Wulongqing formations and large-scale sedimentary features were noted. A total of 2988 fossil specimens were collected in one four-week field season sequentially and independently from ten contiguous siltstone and mudstone layers varying in thickness from 6 to 110 cm. Whole fossils were identified and classified to the phylum level and, where applicable, Brachiopod genera were identified. Faunal relative abundances are based on all the well-preserved fossils, whereas trace fossils and fragmentary and unidentifiable specimens, as well as all shell concentrations, were excluded.

 

Field photographs of the lower Wulongqing Formation at the Shijiangjun section. Yellow upper case letters mark the layers yielding a fossil assemblage in accordance with Figure 2. (a) General view of the lower part of the section. The yellow line on the bottom of the section indicates the lithological contact between the Hongjingshao and Wulongqing formations. (b) Load casts at the bottom of the sandstone deposits. (c) Wavy bedding structure above layer C. (d) Normal graded bedding from fossil-yielding layer D, scale bar: 1 cm. (e) Gutter casts, lenticular bedding and wavy ripples at the Shijiangjun section. (f ) Plan view of gutter casts from fossil-yielding layer F. (g) The simplified palaeoenvironmental reconstruction for the Guanshan Biota from Wuding area. Abbreviations: HJS, Hongjingshao Formation; WLQ, Wulongqing Formation. Chen et al. (2020).

Lithological samples (41 in total) in oriented plaster jackets were collected at intervals from mudstone and sandstone layers through the section. All the samples collected for rock slabs and thin sections were cut and polished at the Shaanxi Key Laboratory of Early Life and Environments, China and revealed the vertical internal organization of the physical and biogenic sedimentary structures. Scanning of the polished slabs was achieved using an Epson V370 photo-scanner at Macquarie University, Australia. Adobe Photoshop was used to digitally improve the visibility (contrast) of the sedimentary and ichnological structures. Sedimentary characteristics, including grain size, lithology, sedimentary structures and vertical bioturbation intensity were recorded. The percentage bioturbation in each sample was evaluated using Adobe Photoshop. The bioturbation area was selected using the lasso tool and recorded through the measurement log in pixels. This was then divided by the total area in pixels to determine the percentage of bioturbation. These percentages were then used within a bioturbation index. All the rock samples and fossil specimens investigated are deposited in the Early Life Institute and the Department of Geology, Northwest University, Xi’an China.

The stratigraphic section is 8 m thick and composed of distinctive intercalated beds of thin to thick (5–60 cm), very fine to very coarse sandstone, siltstone and mudstone. Rare gravels and isolated pebbles occur in sandstone samples S2, S3, S4, S5, S6, S9 and S16, in addition to two layers of purple muddy medium to coarse sandstone (S15 and S16), which contained 3–5% oolite grains. Commonly developed primary sedimentary structures include massive bedding, normal graded bedding, lenticular bedding and wavy bedding. The contacts between the sandstones and mudstones are sharp. The most common local erosion structures include gutter casts, erosional scour and low ripple marks. The measured section has an overall low level of bioturbation, with some highly bioturbated beds occurring in the middle part of the section (3.3–5.3 m) accompanying the only identified trace fossil Teichichnus? isp. Based on lithological, sedimentary and ichnological features, the section is divided into four distinct facies that repeat and cycle throughout the section.

 

Polished slabs of the lower Wulongqing Formation at the Shijiangjun section, with facies classification and sample numbers in parentheses. (a), (b) Slabs of Facies 1 showing wavy laminations, graded lamination, lenticular lamination and erosive base. (c) Silty mudstone without sedimentary structures representing Facies 2. (d)–(g) Massive sandstone deposits representing Facies 3. (d), (e) Poorly sorted, angular to sub-angular clasts with few granules. (f), (g) Highly bioturbated sandstone with glauconite grains. (h) Mudstone without structures (Facies 4). Scale bars 5 mm. Chen et al. (2020).

Facies 1 consists of thinly bedded mudstone with thin to thick laminated siltstone and/or very fine sandstone. The silt and sand grains are medium to well-sorted, mainly angular to subrounded, low to high sphericity with increasing sphericity upsection. Fine to medium sandstone intercalations occur as lenticular and wavy bedding. Laterally discontinuous millimetre-scale (mainly 3–5 mm with some about 1 mm) silt laminations are common. Graded laminations (4–10 mm) manifest either as a sharp horizontal contact or an erosional base (sole marks). The contact between sand and mud is nearly always sharp. Bioturbation is generally indistinct and unidentifiable, with Teichichnus? isp. documented in two samples. The bioturbation index ranges from 0 to 3, with a predominant index of 0–1 (up to 4.89% disturbance). More heavily bioturbated beds exist locally (M5 and M20) with indexes of 2–3 recording up to 40% sedimentary fabric disturbance. The graded laminations and erosive bases suggest deposition from decelerating flows. The medium maturity of the sand/silt laminations probably indicates a certain degree of winnowing and transportation.

The interbedded mudstone and sandstone reflect an alternation of quiet water sediment fallout (low energy) combined with relatively high-energy flows.

 

Photomicrographs of thin sections from the lower Wulongqing Formation at the Shjiangjun section showing four lithofacies types. (a) Medium sorted irregular grains from Facies 1. (b) Graded laminations with an erosional base from Facies 1. (c) Mudstone with low content of well-sorted silt grains from Facies 2. (d) Poorly sorted grains with low sphericity from Facies 3. (e) Common glauconite grains within Facies 3; note the iron oxides within grains (black arrows). (f ) Highly bioturbated sandstone from Facies 3. (g) Poorly sorted sandstone from Facies 3, coarse grains are irregular with low sphericity. (h) Uniform mudstone of Facies 4. All photomicrographs were taken with parallel light except (d), which is under cross-polarized light. Scale bars 1 mm. Chen et al. (2020).

Facies 2 is represented by uniform mudstones with occasional millimetre-scale silt laminations (no more than 1 mm). The silt grains are moderately sorted, angular to subrounded (low content) and of low sphericity. Interestingly, the M10 layer contains a higher concentration of muscovite than any other layer. Fragmentary shelly fossils are often present and are preserved parallel or oblique to bedding, with a particularly high concentration of trilobite fragments documented in layer M25. Bioturbation is rare (BI = 0), with the percentage bioturbation never exceeding 1%.

The absence of rheological surfaces on the silty mudstone packages indicates a relatively low-energy hydrodynamic system. Abundant sub-parallel to oblique Brachiopod and/or Trilobite fragments within the mudstone indicate transportation by currents.

High rates of fallout or other unobservable environmental stressors (e.g. oxygen, salinity or temperature) may be responsible for the relative absence of bioturbation. As a result, the relatively structureless silty mudstone packages are interpreted as deposited from rapid fallout from suspension during quiet periods of fair weather conditions.

Facies 3 consists of very fine to very coarse sandstone with rare granule- to pebble-sized clasts. The granules and pebbles predominately occur in samples S2–S6, S9 and S16. The medium- to very coarse-grained sand beds from the lower and upper part of this section are characterized by very poorly to poorly sorted grains distributed within the intervals 0–2.1 m and 4.6–5.0 m. Coarse grains are mainly angular to subrounded and dominated by low to medium sphericity. Although the very fine- to medium-grained sand beds from interval 2.2–4.2 m are mainly moderately sorted, beds show medium to high sphericity. Two beds (S15 and S16) contain 1–5% elongate ooids. Most of the ooids are oval and few are rounded.

The sandstone beds are either characterised by a homogeneous uniform grain size or high bio-disturbance, which has destroyed the original sedimentary structures. Only levels S7 and S8 show weakly normal graded bedding. Sand beds S11–S15 show a relatively higher content of mud and a higher percentage of bio-disturbance (BI = 2–5). The bioturbation index and biodisturbance reach a peak of BI = 5 and 98.76% within S12, followed by S11 (80.88%) and S14 (76.16%). However, more than half of the sandstones below S11 show scarce or no bioturbation. 

The occurrence of syngenetic glauconite grains within the sandstones of Facies 3 is unique. These grains were identified based on their green colour, random microcrystalline internal texture and aggregate polarisation. They are, in some instances, coated and replaced by iron oxides (mostly hematite and goethite). These grains occur in every sandstone interbed at relatively low contents. The grains are usually medium sorted, subrounded to rounded and of medium sphericity. Although glauconite cannot be used as a specific environmental indicator, it is commonly associated with transgressive systems tracts. Different types of glauconite (i.e. autochthonous, parautochthonous and detrital) can be determined. The glauconite that usually occurs in detrital granular and sand facies lacks a diffuse green pigmentation, which often alternates between glauconite-rich and glauconite-free layers, and can be interpreted to indicate an allochthonous (e.g. parautochthonous or detrital) origin. By contrast, the low compositional and structural maturity of Facies 3, as well as a lack of glauconite in the older Hongjingshao Formation, implies a parautochthonous origin, in which the autochthonous glauconites have been transported a short distance from their original location by waves, storm currents and/or gravity flow processes.

Local observations of Facies 3 show that these sandstones have a low compositional and textural maturity, which suggests that the sediments were deposited with minimal traction and clast collisions from a proximal sediment source. Therefore the clasts retain their immature, angular texture. Storm deposits are generally understood to consist of well-sorted sand with a fining upwards sequence that reflects the waning storm waves. The storm flow usually converts to a turbidity current as the power of the storm flow weakens near the storm wave base, resulting in the suspended mud and gravel depositing together with fine suspended sediments during recessive periods.

The common occurrence of poor bedding and disordered accumulation indicate fairly rapid suspension fall out without winnowing, probably affected by gravity flow deposition in relatively deeper water. The sharp contacts at the lower and upper boundaries between the sandstones and mudstones show that each sandstone layer represents a single event. However, the changing grain size inside the thin sandstone units shows an unstable hydrodynamic environment. Facies 3 is interpreted to have been deposited within lower shoreface zone formed near the storm wave base and was affected by multiple pulses of gravity flows.

Facies 4 represents mudstones with occasional interbedded wisps of silt. The mud layers are considerably thicker (2.5–3 cm) than in other facies. The silt laminations are fairly thin (0.3–1 mm) with sharp erosive bases and a crudely micrograded lower part and structureless upper part. Shelly fossils preserved within Facies 4 are usually parallel to sub-parallel to the bedding plane. The bio-disturbance within Facies 4 is the lowest among the four facies, only up to 0.15%, resulting in a low bioturbation index (BI = 0).

These sedimentary features, along with the soft tissue preservation associated with Facies 4, suggest a mainly rapid deposition (obrution) of suspended muds settling from weak storm flows in a relatively low-energy environment.

Thousands of well-preserved fossils spanning six key animal groups (2988 specimens in total) were collected from the lower Wulongqing Formation at the Shijiangjun section during one four-week field season. The taxa include Brachiopods, Arthropods, Hyoliths, Priapulids, Vetulicolians and Anomalocaridiids in descending order of rank abundance. All these taxa are also found in the Wulongqing Formation from the Kunming and Malong areas. Brachiopods, arthropods and hyoliths form the three main components, with up to 98.9% of the total number of specimens. Even though the Anomalocaridiids, Vetulicolians and Priapulids are rare in this section, they are very important elements of Cambrian Burgess Shale-type Lagerstätten. Four genera of Organophosphatic Brachiopods, including Neobolus, Eoobolus, Westonia, Linnarssonia, and two calcareous taxa (Kutorgina and Nisusia) occur throughout the section. Neobolus is the most abundant genus (40.2%), followed by Eoobolus (28.9%) and Westonia (27.2%). However, Arthropods remain the most diverse group, composed of Trilobites, Bradoriids, Guangweicaris, Panlongia, Isoxys, Tuzoia and Leanchoilia. Among these, Trilobites are the most abundant taxon (82%).

 

Pie charts of relative abundance for the Malong Fauna and the Guanshan Biota. Note the rising relative abundance of Brachiopods in the Guanshan Biota. Chen et al. (2020).

Fossil data from every mudstone layer was obtained during four weeks of intensive fieldwork in 2018. The fossil composition within assemblages A and B is similar, consisting of five animal groups, while faunal diversity decreases in assemblages C–F. This is followed by an increased diversity associated with faunal assemblages G–J. Faunal assemblage I has the highest diversity, with almost all taxa known from the entire section concentrated in this assemblage. Assemblage F has the greatest abundance of fossils (748 specimens) accounting for 25% the total number of individuals, followed by assemblages C, B, G and J.

The relative abundance of individual specimens from ten sampling layers was obtained to gauge the baseline assemblage structure. Assemblages A and B are dominated by Arthropods, accounting for 63.6 and 59.8%, respectively. Brachiopods dominate all other assemblages from layers C–J, with some fluctuation of composition in the relative abundance between Brachiopod taxa. The abundance of brachiopods reaches a peak within assemblage F. Hyoliths, a common early Cambrian group, occur throughout the entire section, except for assemblage G. Anomalocaridiids, Vetulicolians and Priapulids are interspersed irregularly within the assemblages.

The relative abundance of six genera of Brachiopods throughout the section is very instructive. Assemblage A is composed, almost equally, of three genera (Neobolus, 36.4%; Eoobolus, 36.4%; and Linnarssonia, 27.2%), whereas assemblage B contains a higher proportion of Neobolus (51.1%), with the relative abundance of the remaining two taxa 26.1 and 22.8%, respectively. Westonia occurs as a small proportion of assemblage C, whereas Neobolus and Eoobolus together exceed 97%. Assemblages C and D are mainly composed of Eoobolus (20.5 and 62.8%, respectively) and Neobolus (77.3 and 26.7%, respectively) with minor Westonia. By contrast, Westonia reaches a higher relative abundance (26.2%) in assemblage F. Eoobolus dominates assemblages G and H (52.4 and 64.2%, respectively), where Westonia also reaches a higher proportion of the assemblage (44.7% in G). Assemblages I and J are both dominated by Westonia, with 61.5 and 88% relative abundance, respectively; Eoobolus (24 and 9.6%, respectively) ranks second in these assemblages. The rare calcareous Brachiopods Kutorgina and Nisusia are restricted to the upper part of the section in assemblages I and J.

 

Stratigraphic fluctuation in the relative abundance of the community at the (a) phylum level and (b) Brachiopod genus level from the Shijiangjun section. Chen et al. (2020).

The lower part of the Wulongqing Formation (0–6 m) at the Shijiangjun section also contains distinctive Brachiopod and Trilobite fossil concentrations. The concentrations preserved in coarser sandy deposits are highly fragmented (although also fragile and thin) and moderately wellsorted, which indicates a relatively high level of energy and transportation. Some well-preserved shell concentrations are also preserved within thin mud beds (e.g. Facies 1 and 4), occasionally restricted to single bedding planes, and in a relative sense these thin shells are characterised by low levels of fragmentation, poor sorting, low to medium disarticulation, and occur sub-parallel to bedding planes with a high ratio (over 50%) of conjoined Brachiopod shells with more or less soft tissue preservation. These taphonomic proxies indicate a relatively rapid obrution deposit and minimal transportation. The shell concentrations from the Shijiangjun section are either monospecific or paucispecific, dominated by Brachiopods or Trilobites. These concentrations are nearly always restricted to specific layers. For example, abundant Palaeolenus are exclusively found within layer M6 in assemblage B, whereas a concentration of Linnarssonia shells is known within layer M3 in assemblage A. The Brachiopod concentrations from assemblage F are most abundant and mainly composed of monospecific layers of Neobolus or Westonia, respectively. The Eoobolus and Westonia shell concentrations extend to the upper part of the section. Throughout the section, Brachiopod concentrations are completely restricted to Facies 1 and 2, whereas trilobite concentrations are mainly associated with Facies 4, which is restricted to assemblage B.

 

Exquisitely preserved soft-bodied fossils from the lower Wulongqing Formation at the Shijiangjun section. (a) Brachiopod Linnarssonia concentration from assemblage A (ELI-SJJ-001). (b) Trilobite Palaeolenus concentration from assemblage B (ELI-SJJ-002). (c), (d) Brachiopods Neobolus and Westonia concentrations from assemblage F (ELI-SJJ-003, ELI-SJJ-003-2). (e) Brachiopod Neobolus with well-preserved parasitic Tubeworms, indicated by arrows (assemblage B, ELI-SJJ-004). (f ) Brachiopod Westonia preserved with mantle canals (assemblage F, ELI-SJJ-005). (g) Rare Brachiopod Nisusia sp. (assemblage J, ELI-SJJ-006). (h) Well-preserved coiled Palaeoscolecidan (assemblage F, ELI-SJJ-007). (i) Posterior part of an indeterminate vetulicolian (assemblage A, ELI-SJJ-008). ( j) Trilobite Palaeolenus preserved with the rare digestive system (assemblage B, ELI-SJJ-009). Scale bars: (a), (e)–(h) and (j) 2 mm; (b)–(d), (i) 1 cm. Chen et al. (2020).

Remarkable soft tissue preservation occurs in all assemblages except D and E, demonstrating the high preservation potential within facies at the Shijiangjun section of the Wulongqing Formation in the Wuding area. Tube-dwelling organisms encrusting to Neobolus shells (with exceptionally preserved setae and soft viscera) are fairly common within the lower part of the section within mudstone beds (layers A, B, C and F). Abundant specimens of Westonia display high-quality soft tissue preservation from assemblage F, including setal fringes and mantle canals. Palaeoscolecidan Worms, as an important component of lower Paleozoic soft-bodied assemblages, were found throughout the section, except for assemblage C. Relatively rare Vetulicolians occur at the base and in the upper part of the section (assemblages A, B, I and J). Anomalocaridiids are the rarest element in the section, only preserved as isolated frontal appendages in assemblages I and J. The rare oldest known digestive system of Trilobites have also been preserved in the Wuding area, but only in assemblage B.

Heterolithic successions consisting of sandstone beds interbedded with mudstones are usually deposited below the fair weather wave base and above the storm wave base. These beds are commonly described as tabular and often show abundant erosive gutter casts. The alternation of mudstone (Facies 1, 2 and 4) and sandstone (Facies 3) layers, in addition to graded lamination/bedding, wavy bedding, ripple marks and gutter casts from the Shijiangjun section, suggests a depositional environment close to the storm wave base, which underwent multiple depositional events and episodic cycles.

Previous studies have interpreted the sedimentary environment associated with the Guanshan Biota as mainly offshore transition with common storm events, which is comparable with the Cambrian Stage 4 Emu Bay Shale from Australia. However, typical storm-generated structures such as hummocky cross-stratification, an indicator of oscillatory combined flows reflecting deposition under high-energy storm conditions are absent in the Wuding succession.

The occurrence of erosive bases, ripple marks, wavy bedding, fine-graded bedding, gutter casts and multiple massive fine to coarse deposits indicates a complex hydrodynamic environment, with less frequent waves and distal storms. Periodic subaqueous gravity flows resulted in the deposition of distinctive centimetre-scale sandstone interbeds (Facies 3) at the Shijiangjun section. Hence the sedimentary environment of the lower Wulongqing Formation in the Wuding area is largely the result of fluctuating wave energy, distal storms and gravity flows.

The centimetre-scale conglomerates characterised by high sphericity reported from the Wulongqing Formation at Malong and Kunming represent high-energy channels, probably proximal to the shoreface. The absence of basal conglomerates and the occurrence of medium to very coarse sandstones with few granules at the base of the Wulongqing Formation in the Wuding area suggest a relatively deeper and low-energy clastic sedimentary environment than that in the Malong and Kunming areas, although this remains to be tested because detailed continuous successions of the Wulongqing Formation have not been studied sedimentologically. Overall, the depositional environment here is interpreted as offshore to lower shoreface and the offshore zone, which slightly extends below the storm wave base.

The baseline time series of the fossil data recovered from the lower Wulongqing Formation at the Shijiangjun section reveals a unique transition in the structure of the benthic community over time. The relative abundance of six key Animal groups, including six Brachiopod genera, from ten sampled layers demonstrates gradual replacement, overturn and fluctuation in the faunal composition. Although Arthropods dominate the base (0–1.1 m) of the section (assemblages A and B), the proportion of Brachiopods gradually increases, replacing Arthropods as the dominant fauna in assemblages C–J, reaching peak abundance (97.99%) within assemblage F. Although there is a fluctuation in the relative abundance of Brachiopods through assemblages G–J (c. 60–80%), Arthropods maintain a relatively low, but stable, percentage.

There is no doubt that Trilobites dominated early Cambrian benthic communities in terms of diversity and abundance, which is demonstrated well in the older Chengjiang Lagerstätte and the Malong Fauna. The latter is characterized by extremely abundant and diverse Trilobites yielding from the underlying Hongjingshao Formation. However, detailed fossil data from the Guanshan Biota in Wuding and Malong areas reveals a community structure that is unique for early Cambrian Konservat-Lagerstätten, with Brachiopods dominating the benthic community in abundance, if not diversity, and often forming distinctive concentrations of shell beds in the lower Cambrian Stage 4 of the Wulongqing Formation. The ecological transition from Trilobite to Brachiopod-dominated communities occurs widely across shallow marine clastic environments across the South China Platform, coinciding with well-documented transgression events during Cambrian Age 4. Thus Organophosphatic Brachiopods diversify and become superabundant across the broad ‘shallow’ shelf of the Yangtze Platform during the final stage of the Cambrian Explosion. The rise of Organophosphatic Brachiopods as the numerically dominant element in the lower Cambrian Stage 4 Wulongqing Formation is the oldest Brachiopod-dominated soft substrate community known in the fossil record and represents a precursor to more complex community tiering and Brachiopod-dominant benthic communities during the Great Ordovician Biodiversification Event.

The Brachiopods recovered from the section include Lingulides (Eoobolus, Neobolus and Westonia), an Acrotretide (Linnarssonia) and calcareous Kutorginides (Kutorgina and Nisusia). Lingulides occur in high abundance and also form many shell concentrations within several assemblages. The number of Brachiopod concentrations (at least ten thin mud beds) far exceeds those produced by Trilobites (only one mud bed). The composition of |Brachiopod taxa within each assemblage shows a rapid transition through time. Neobolus is predominant in the lower part of the section (assemblages A–C, E and F), with Eoobolus (Lingulides) and Acrotretides common, but subordinate. The relative abundance of the Acrotheloid Brachiopods, earlier referred to as ‘Westonia’ gubaiensis, increases gradually up-section, replacing, in part, the Lingulides (Eoobolus and Neobolus) and Acrotretides. This is partly attributed to the fact that the Brachiopods of Eoobolus and Linnarssonia had a much smaller shell (about 2–5 mm in maximum length) than Westonia. In addition, Westonia has a very wide and circular shell in outline, which is potentially adapted to the shallowing seawater environment. In general, the Linguliform (e.g. Lingulides and Acrotretides) Brachiopods show a strong control on assemblage dominance, whereas calcareous forms (Kutorginides) remain rare.

Fossil concentrations, although common throughout geological time, are rarely reported from Burgess Shale-type Lagerstätten. The dominance of Brachiopods within the Guanshan Biota, compared with other Cambrian Lagerstätten, is unique. The in situ preserved Brachiopod concentrations in the Wuding area also occur in the Malong and Kunming areas, which indicates a wide geographical distribution (about 6000 km²) after the rapid transgression at the base of the Wulongqing Formation.

Overall, the fossil data show that Brachiopods quickly replaced Arthropods as the dominant fauna following a transgression that led to the deposition of the Wulongqing Formation at Wuding. Different brachiopod genera dominated different assemblages and, in places, formed distinctive shell concentrations.

The Guanshan Biota is an exceptionally preserved Konservat-Lagerstätte, uniquely characterised by brachiopod-dominated early Cambrian communities, substantially different from the Arthropod-dominated Konservat-Lagerstätten such as the Chengjiang and Burgess Shale biotas. Although the preservation of soft tissues within biomineralised and sclerotised exoskeletons is common, which is at least partly attributable to the high number of Brachiopods, Trilobites and Hyoliths, completely soft-bodied organisms (e.g. Ctenophores) are absent in the Shijiangjun section, which is similar to the Ordovician Fezouata Biota. This phenomenon is possibly related to preservation bias because the Brachiopods, Trilobites and Hyoliths are more resistant to decay and much more readily preserved within this Konservat-Lagerstätte, which might lead to an underestimation of the diversity of the Guanshan Biota in the Wuding area. The lack of completely soft-bodied taxa may be due to the lack of an exaerobic preservational trap that typifies the Burgess Shale-type deposits.

The relatively shallow sedimentary environment (lower offshore or offshore) of the Guanshan Biota also separates it from most other Cambrian Lagerstätten worldwide except, perhaps, for the early Cambrian Emu Bay Shale from Australia, which is interpreted to have been deposited in a nearshore micro-basin setting adjacent to an active tectonic margin that generated continual syndepositional faulting and slumping. The Guanshan Biota is also comparable with the Ordovician Fezouata Biota, both in terms of depositional environment and shelly faunal composition. The latter was deposited mainly in an offshore to lower shoreface setting.

Gravity, traction and turbiditic flows are responsible for the transitions from Arthropod- to Brachiopod-dominated assemblages from the lower part of the Wulongqing Formation at the Shijiangjun section. The depositional environment between the fair weather wave base and the storm wave base is usually affected by frequent event flows, such as oscillatory and gravity flows, which helps to mix oxygen-enriched surface water with stagnant bottom water, providing favourable nutrient-rich conditions for the development of the benthic community. Transportation from a nearby source, rapid fall out from suspension and the resuspension of seston provides a high nutrient load for suspension feeders such as Brachiopods to flourish.

The limited amount of bioturbation throughout most of the section seems to indicate conditions unfavourable for burrowing, resulting from high turbidity, high or low salinity, or the relatively low oxygen content, perhaps explaining the dominance of relatively small, physiological simple filter-feeding Brachiopods. The increase in the bioturbation index in the middle part of the section (3–6 m above the basal contact) is coincident with assemblages G–I, indicating more favourable conditions, probably a result of the relatively shallower depositional environment or fluctuating oxic conditions. The frequent overturn of fossil assemblages, especially Brachiopods, may be attributed to frequent environmental fluctuations and the episodic input of coarser sediments, which probably periodically interrupt the benthic suspension assemblages.

Detailed analysis of the sedimentology, lithology and structures facilitates the identification of distinct lithofacies associated with transgressive systems tracts that directly affected the composition, diversity and relative abundance of faunal assemblages in the transition from the Hongjingshao to the Wulongqing deposits. Microfacies analysis, the degree of bioturbation and the faunal composition at the lower part of the Wulongqing Formation provide a new understanding of how fluctuations in the depositional environment influenced the faunal overturn in the Guanshan Biota across the Yangtze Platform in eastern Yunnan.

This is the first detailed report of the lithofacies, depositional environments and associated relative faunal abundance in the Cambrian Age 4 Guanshan Biota. The new Shijiangjun section through the basal part of the Wulongqing Formation in the Wuding area, eastern Yunnan reveals fossil assemblages composed of six Bilaterian groups (Brachiopoda, Arthropoda, Hyolitha, Priapulida, Vetulicola and Anomalocaridiids). Detailed sedimentological, lithological and ichnological characteristics of the section indicate that: (1) hydrodynamic conditions are fluctuating, with episodic changes in energy and current regimes producing periodically coarse sand beds (Facies 3); (2) the sediments are derived from a relatively nearby source and accumulated rapidly; (3) the environment is affected by multi-period hydrodynamic events, such as storm and gravity flows forming obrution deposits; and (4) the overall sedimentary environment in the Wuding area represents a deeper offshore to lower shoreface than the Wulongqing Formation outcropping in the Malong and Kunming areas.

The community transitioned from Arthropod- to Brachiopod-dominated for the first time at the base of the Wulongqing Formation in the Shijiangjun section. Within the Brachiopod communities, a lingulate-dominated assemblage transitioned to an Acrotheloid-dominated assemblage with the new occurrence of calcareous Kutorginides up-section. The detailed study and documentation of this transition provides a better understanding of the differences in faunal composition and overturn between the Malong Fauna and Guanshan Biota. The unstable sedimentary environment with periodically sandy depositional inputs and muddy obrution deposits is probably closely associated with the observed succession of community assemblages. Brachiopods from the Guanshan Biota generally show a preference for such a fluctuating environment and adapt well to this environmental setting during the final stage of Cambrian evolutionary radiation.

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